It is well known to use specialized circuits, commonly called choppers, to periodically break a direct current signal and thereby supply an alternating current having the same voltage and amperage as the direct current signal. By use of these choppers it is possible to amplify a direct current or continuous signal by the use of conventional alternating current amplifiers. This permits the main parameters of the signals to be accurately measured without the use of direct current amplifiers because, as is well known, the disadvantages of direct current amplifiers are many. Various schemes have been proposed to accomplish this chopping function; for example, hard switching by mechanical relays driven by a rotating disc between a photo cell and light source to produce the desired square wave chopping, and soft switching of the direct current wave by the use of a transistor or a silicon controlled rectifier. At the present time a major problem which is present in many choppers is the existence of parasitic thermo-electric noise. In an attempt to eliminate this noise, nonmechanical choppers operating at extremely low temperatures have been designed utilizing the principle of superconductivity, which is a property of many conductors, including metals. In these superconductivity systems, chopping is provided by making a resistance alternately superconductive and nonsuperconductive. Such alternating between conductive states may be achieved by the use of a magnetic field which operates to cancel the superconductivity property, or by a periodically varying thermal means. Such thermal means may operate by simply heating and cooling a resistance periodically. It has also been suggested that the resistance could be subjected to electro-magnetic radiation, such as light, and that it would absorb the radiation and transform it into heat. In this way the superconductivity property could be varied to provide the required chopping. However, devices and circuits of this type have proven to be extremely complicated and do not provide the necessary high impedence during the nonsuperconductivity periods which is required in many applications to prevent loading.
It is therefore an object of the present invention to provide a chopper circuit having a superconductivity control means which utilizes a light beam.
It is a further object of the present invention to provide a chopper circuit having low parasitic thermo-electric noise.
It is a still further object of the present invention to provide a superconductivity chopper circuit which is simple and relatively small and utilizes no external optical apparatus.